Air cooled heat transfer equipment is used in a variety of industrial settings. For example, air cooled condensers are used in large power plants as alternatives to water-cooled systems. Such condensers usually include elongated tubes of elliptical or oblong cross section arranged in a horizontal, vertical, or A-frame construction. The elongated tubes include high thermal conductivity fins which are brazed or otherwise attached to the exterior of the tubes. The condensers are designed for life expectancies of at least 30 years and therefore the tube and fin assemblies must be highly corrosion resistant. The tube and fin assemblies are also used in heat transfer equipment other than condensers.
The first generation of such tube and fin assemblies included carbon steel fins and tubes which were galvanized for corrosion protection. The galvanizing method entailed several cumbersome processes, for example the finned tube had to undergo a number of steps including cleaning, pickling, and rinsing, before galvanizing could take place. Unlike standard galvanization methods, methods for manufacturing galvanized steel assemblies required higher zinc bath temperature and larger bath sizes. These facilities are not readily available globally. Furthermore, zinc is considered as a heavy metal and can leach into ground water under the acid rain conditions.
Because of these and other problems with the galvanizing method, a second generation of tube and fin assemblies which eliminated the galvanizing step was developed. Such second generation assemblies had aluminum-coated carbon steel tubes brazed to aluminum fins. These second generation assemblies were very difficult to manufacture due in part to the complexity of brazing of dissimilar metals. Brazing such dissimilar metals requires very special brazing materials and techniques. The base tube has to be clad first with an aluminum based alloy. Tubes have to be coated prior to fin assembly. The fin material has to be of the same material as the clad on the base tube. In addition, a flux of different material than the coating has to be applied to the tube and fins prior to the brazing process.
Historically, the term “brazing” has meant the joining of two components using brazing material comprising brass. The term brazing, as used in this art, does not refer to conventional brazing techniques using brass or a coating of a flux comprising brass.
Various attempts have been made to improve the brazing materials and techniques for such aluminum finned tube assemblies. One recent patent, Korischem, et al., U.S. Pat. 6,848,609, assigned to GEA Energietechnik GmbH, describes a fluxing technique with a fluxing agent of cesium-aluminum tetrafluoride applied in lines onto the crests of at least one of the ends of the finned structure, placing the structure upon a broad side of a flat tube coated with a zinc/aluminum alloy, and subsequently joining in a brazing furnace in the presence of an inert gas at a temperature of between 370° and 470° C. to produce the assembly.
Another attempt at solving the problem of galvanizing very large steel fin-steel tube assemblies has been to reduce the tube size which allowed the assemblies to be successfully galvanized
There has been a long-felt need in the art to provide large tube and fin assemblies for air cooled condenser systems which avoid the problems associated with prior galvanizing techniques and prior aluminum fin techniques.